Exciton-phonon coupling and power dependent room temperature photoluminescence of sulphur vacancy doped MoS2via controlled thermal annealing

Via experiments performed by varying the doping level of single-layer mechanically exfoliated MoS₂via post exfoliation thermal annealing in the 200-300 °C temperature range, we study the power dependent room temperature (RT) photoluminescence (PL), which is dominated by A type excitons. The PL spectral yields of the as-exfoliated and annealed samples show sub-linear behaviour as a function of the excitation laser power. The PL signal of the 200 °C annealed sample is dominated by the charged exciton (trion) related peak, while the PL signal of the 300 °C annealed sample is dominated by the neutral exciton related peak, and the PL spectral weight of excitons is tunable in this temperature range. The PL signal increase due to annealing and the intensity ratio of the A type excitons are related, showing a hyperbolic tangent trend. We directly quantitatively demonstrate the one-to-one correlation of the RT resonant Raman (632.8 nm) integrated spectral intensity with the corresponding PL spectral yield, providing experimental evidence of the exciton-phonon coupling effect. The in-plane E_2g¹ Raman mode exhibits strong coupling with A excitons, while the out-of-plane A_1g Raman mode does not. This is an indication of the in-plane spatial symmetry of the A excitons.

Simple Chemical Treatment to n-Dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics

The optical and electrical properties of monolayer transition-metal dichalcogenides (1L-TMDs) are critically influenced by two dimensionally confined exciton complexes. Although extensive studies on controlling the optical properties of 1L-TMDs through external doping or defect engineering have been carried out, the effects of excess charges, defects, and the populations of exciton complexes on the light emission of 1L-TMDs are not yet fully understood. Here, we present a simple chemical treatment method for n-dope 1L-TMDs, which also enhances their optical and electrical properties. We show that dipping 1Ls of MoS₂, WS₂, and WSe₂, whether exfoliated or grown by chemical vapor deposition, into methanol for several hours can increase the electron density and also can reduce the defects, resulting in the enhancement of their photoluminescence, light absorption, and the carrier mobility. This methanol treatment was effective for both n- and p-type 1L-TMDs, suggesting that the surface restructuring around structural defects by methanol is responsible for the enhancement of optical and electrical characteristics. Our results have revealed a simple process for external doping that can enhance both the optical and electrical properties of 1L-TMDs and help us understand how the exciton emission in 1L-TMDs can be modulated by chemical treatments.